This invention relates in general to polyamide-imide resins and, more specifically, to improved methods for making high molecular weight, flame resistant modified polyamide-imide foams.
Prior U.S. Pat. Nos. 4,161,477, 4,183,838 and 4,183,839 disclosed and claimed certain polyimide compositions and methods of producing adhesive and coating products using those compositions.
The coating and adhesive compositions described in these patents are made by reacting an aromatic tetracarboxylic acid dianhydride with an oxoimine to produce a bisimide, which is then mixed with an inert solvent and a diamine, producing a viscous fluid containing an intimate, unpolymerized mixture of N-substituted cyclic bisimide dicarboxylic acid and diamine which is capable of being converted to a high molecular weight polymer by the application of heat.
When coated on a surface or layered between two surfaces and heated to a temperature in the range of about 177.degree. to 316.degree. C. a tough highly adherent coating or adhesive results. This material was not, however, suitable for use in applications requiring a cellular or foam material, since conventional agitation foaming and addition of known blowing agents add to process costs and complexity and are not entirely effective at the relatively high polymerization temperature required.
A method of producing polyimide foams which overcomes many of these problems is described in our earlier U.S. Pat. Nos. 4,394,464 and 4,426,463. In that method an aromatic dianhydride is reacted with an oxoimine at a temperature of about 190.degree. C. to produce an N-substituted aliphatic imide. The resulting product is cooled below about 70.degree. C. and dissolved in a reactive solvent esterifying agent and heated to reflux for at least 60 minutes to esterify the imide. The material is dried, ground to a powder, then heated to at least 200.degree. C. to cause foaming. An excellent, flexible foam results.
Recently, in our co-pending U.S. patent application Ser. No. 678,992, filed Dec. 6, 1984, we disclosed and claimed another foam producing process which could be varied during processing to produce foams varying from almost entirely polyimide-amide to mixed polyimide and polyimide-amide. An aromatic dianhydride is reacted with an oxoimine in alcohol at 60.degree. to 120.degree. C. followed by adding a diamine, drying to a powder, then heating to melt and foam. The type of polymer and, accordingly, the corresponding physical characteristics depend upon the foaming temperature, in the overall 230.degree. to 315.degree. C. range. While foam physical properties could be easily selected, this process requires high processing temperatures and several complex operations.
While the prior art techniques often produced excellent foams, those methods are undesireably complex, requiring a number of sequential steps to be carefully performed at varying, relatively high temperatures. This tended to result in high energy costs and sometimes varying product quality due to processing variations.
Thus, there is a continuing need for improved methods of producing foams having processing simplicity and low temperature processing, together with superior flexibility and flame resistance.